Why call to function through a constexpr function pointer is not inlined?

Question

Why call to function through a constexpr function pointer is not inlined?

HD86 26

When the following code is compiled with the latest MSVC compiler, the call to the function foo is not inlined. Is there a valid reason for this, or is it a compiler deficiency?

template <auto V>
struct Wrapper
{ 
    static constexpr decltype(V) VALUE = V; 
};

void foo(int i) { std::cout << i; }

template <typename T>
void bar(T) { T::VALUE(8); }

int main()
{
    bar(Wrapper<&foo>());
}

David Lowndes 4,726 Reputation points

2020-10-08T07:56:34.387+00:00

If you think it's a deficiency (perhaps some other compilers do manage to inline it and in your real world scenario that proves a big advantage), submit a bug report using VS's Report a Problem facility.

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David Lowndes 4,726 Reputation points

2020-10-08T07:56:34.387+00:00

If you think it's a deficiency (perhaps some other compilers do manage to inline it and in your real world scenario that proves a big advantage), submit a bug report using VS's Report a Problem facility.

Answer 1

Viorel 122.6K

Try using this linker option (in Project Properties) — Link Time Code Generation: “Use Link Time Code Generation (/LTCG)”.

How did you check the inlining results?

HD86 26 Reputation points

2020-10-08T13:31:40.037+00:00

This solves the problem, but it raises another problem which is that the option /LTCG:INCREMENTAL apparently produces less optimal code when function pointers are used.
I check the inlining result by hitting "Start Debugging" then "Go To Disassembly."
Viorel 122.6K Reputation points

2020-10-08T14:00:29.86+00:00
The inlining seems to also work in this case:

Whole Program Optimisation: “No”

Link Time Code Generation: /LTCG or /LTCG:incremental.

Therefore, it appears that Whole Program Optimisation, when enabled, delegates some tasks to Linker, which does a better job without incremental things, maybe slower.

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Why call to function through a constexpr function pointer is not inlined?

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